Heat exchanger including twisted tubes

ABSTRACT

A heat exchanger comprising a shell having a first end and a second end, a first end plate and a second end plate defining a first volume with the shell that receives a first fluid therein, and at least one heat exchanger tube having a first end affixed to the first end plate and a second end affixed to the second end plate. The at least one heat exchanger tube extends through the second volume and including a cross-section defined by a plurality of lobes extending radially outwardly from the longitudinal center axis thereof. A first fluid passes through the first volume of the shell and a second fluid comprising a hot combustion gas passes through the at least one heat exchanger tube. The cross-section is constant along an entire length of the at least one heat exchanger tube.

FIELD OF THE INVENTION

The present invention relates generally to shell and tube type heat exchangers.

BACKGROUND OF THE INVENTION

Shell and tube type heat exchangers are used to transfer heat from a first fluid and a second fluid, such as from a hot combustion gas to water, etc. A typical shell and tube type heat exchanger includes a plurality of elongated, cylindrically-shaped heat exchanger tubes that are disposed within the shell and are substantially parallel to the shell's longitudinal center axis. In a basic heat exchanger, the heat exchanger tubes may make only one pass through the shell. However, in more complex heat exchangers, the heat exchanger tubes may make multiple passes within the shell. A combustion chamber in which hot gasses are produced by the combustion of fuels is provided at a first end of the shell. A blower may be used to move the hot combustion gasses through the plurality of heat exchanger tubes from the first end to the second end of the shell, thereby passing through the portion of the shell in which the second fluid, e.g. water, is contained. The shell side of the heat exchanger is provided with an inlet for the second fluid, as well as an outlet that allows the second fluid to exit the heat exchanger after the heating process. Existing heat exchangers often include heat exchanger tubes having a circular cross-section.

The efficiency of heat transfer between the first fluid in the heat exchanger tubes and the second fluid in the shell side of the heat exchanger may be a complicated function that depends of the characteristics of the fluids, on the characteristics of the heat exchanger tubes, and on the characteristics of fluid movement relative to both the inside and outside surfaces of the heat exchanger tubes. One relevant characteristic is tube surface area. For a given amount of combustion gas, a heat exchanger tube's ability to exchange heat varies directly with the tube's surface area. In a cylindrical heat exchanger tube, the tube's inner and outer surface areas depend upon the tube's inner and outer diameters. Within the inner diameter, cylindrical heat exchanger tubes may exhibit substantially laminar flow over a substantial length of the tube. As should be understood, laminar flow impedes heat transfer across the wall of the tube in that as the temperature differential between the flow layers adjacent the inner surface and the outer surface of the tube wall decreases, fluid inside the tubes does not mix to thereby allow fluid toward the tube's center to participate in the heat exchange. In order to improve the efficiency of the heat exchanger, it is desirable to maximize the heat transfer rate across the walls of the heat exchanger tubes. Increasing both the inner and outer surface areas of the heat exchanger tubes for a given length tube may increase the heat transfer rate across the walls of the tubes. As well, directing fluid flow past and through the heat exchanger tubes in a desired flow pattern may increase the heat transfer rate.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with the description, serve to explain the principles of the invention.

SUMMARY OF THE INVENTION

The present invention recognizes and addresses considerations of prior art constructions and methods.

According to one embodiment, a heat exchanger includes a shell having a first end and a second end, a first end plate and a second end plate disposed adjacent the first end of the shell and the second end of the shell, respectively, the shell and the first and second end plates defining a first volume configured to receive a first fluid therein, and at least one heat exchanger tube having a first end affixed to the first end plate and a second end affixed to the second end plate, the at least one heat exchanger tube extending through the first volume and including a cross-section taken transverse to its longitudinal center axis that is defined by a plurality of lobes, each lobe extending radially outwardly from the longitudinal center axis thereof. The at least one heat exchanger tube includes inner and outer surfaces defined by a simultaneous rotation and translation of the cross-section along the longitudinal center axis, and the cross-section is constant along an entire length of the at least one heat exchanger tube between the first and the second end plate.

In a further embodiment, a heat exchanger includes a shell having a first end, a second end, a first volume configured to receive a first fluid, a first end plate disposed at a first end of the shell, the first end plate defining a first aperture of a first shape, a second end plate disposed at a second end of the shell, the second end plate defining a second aperture of a second shape and at least one heat exchanger tube having a first end affixed to the first end plate at the first aperture and a second end affixed to the second end plate at the second aperture. The at least one heat exchanger tube extends through the first volume and includes a cross-section taken transverse to its longitudinal center axis that is defined by at least three lobes, each lobe extending radially outwardly from the longitudinal center axis thereof, the at least one heat exchanger tube defining a second volume. The cross-section is constant along an entire length of the at least one heat exchanger tube between the first end plate and the second end plate, and the first shape and the second shape of the first end plate and the second end plate, respectfully, are the same shape as the cross-section of the at least one heat exchanger tube.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with the description, serve to explain one or more embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended drawings, in which:

FIG. 1 is a partial cross-sectional view of a heat exchanger including a plurality of twisted heat exchanger tubes in accordance with an embodiment of the present invention;

FIG. 2 is a cross-sectional view of the heat exchanger shown in FIG. 1, taken along line 2-2;

FIG. 3 is a perspective end view of a twisted heat exchanger tube of the heat exchanger shown in FIG. 1;

FIG. 4 is a side view of the twisted tube shown in FIG. 1;

FIG. 5 is a cross-sectional view of the twisted heat exchanger tube shown in FIG. 4, taken along line 4-4; and

FIG. 6 is a cross-sectional view of a twisted heat exchanger tube for a heat exchanger in accordance with an alternative embodiment of the present invention.

Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention according to the disclosure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference will now be made in detail to presently preferred embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation, not limitation, of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope and spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents.

As used herein, terms referring to a direction or a position relative to the orientation of the heat exchanger, such as but not limited to “vertical,” “horizontal,” “upper,” “lower,” “above,” or “below,” refer to directions and relative positions with respect to the heat exchanger's orientation in its normal intended operation, as indicated in FIGS. 1 and 2 herein. Thus, for instance, the terms “vertical” and “upper” refer to the vertical direction and relative upper position in the views of FIG. 1 and should be understood in that context, even with respect to a heat exchanger that may be disposed in a different orientation.

Further, the term “or” as used in this disclosure and the appended claims is intended to mean an inclusive “or” rather than an exclusive “or.” That is, unless specified otherwise, or clear from the context, the phrase “X employs A or B” is intended to mean any of the natural inclusive permutations. That is, the phrase “X employs A or B” is satisfied by any of the following instances: X employs A; X employs B; or X employs both A and B. In addition, the articles “a” and “an” as used in this application and the appended claims should generally be construed to mean “one or more” unless specified otherwise or clear from the context to be directed to a singular form. Throughout the specification and claims, the following terms take at least the meanings explicitly associated herein, unless the context dictates otherwise. The meanings identified below do not necessarily limit the terms, but merely provided illustrative examples for the terms. The meaning of “a,” “an,” and “the” may include plural references, and the meaning of “in” may include “in” and “on.” The phrase “in one embodiment,” as used herein does not necessarily refer to the same embodiment, although it may.

Referring now to FIGS. 1 and 2, a shell and tube heat exchanger 100, for example a fire-tube boiler, in accordance with the present disclosure includes a vertically oriented, generally cylindrical shell 102, a first end plate 116 disposed within a first end 104 of shell 102, a second end plate 122 disposed in a second end 106 of shell 102, and a plurality of elongated twisted heat exchanger tubes 140 disposed within shell 102 such that the tubes' elongation dimensions are all substantially parallel to the elongation dimension (e.g. a longitudinal or symmetrical center axis) 114 of heat exchanger 100 (and, more particularly, to the elongation direction or center axis of a volume of the enclosed tank defined by shell outer wall 102 and end plates or walls 116 and 122). A combustion chamber 128 is disposed in first end 104 of shell 102, and is defined in part by first end plate 116. A burner 134 is disposed within combustion chamber 128, and a blower 136 is in fluid communication with combustion chamber 128. An outlet chamber 138 is disposed within second end 106 of shell 102, and formed in part by second end plate 122. The plurality of twisted heat exchanger tubes 140 allows fluid communication between combustion chamber 128 and outlet chamber 138. Note, in alternate embodiments, heat exchanger 100 may be oriented such that its longitudinal or symmetrical center axis 114 is substantially horizontal rather than substantially vertical.

First end plate 116 defines a plurality of entry apertures 118. The shape of each entry aperture 118 is configured to correspond with the cross-section of an end of a corresponding heat exchanger tube 140. As shown, each entry aperture 118 may be considered to be defined by the intersection of two elongated ovals or two slots (where a slot is considered to have the same length and rounded ends of the oval but with flattened sides instead of the oval's curved sides). Accordingly, if the four inner corners of apertures 118 are considered to define a circle, then the aperture defines four projections, or lobes, 152, 154, 156, and 158 extending from that theoretical circular cross-sectioned or cylindrical surface, with a respective gap that extends outward from each corner between and separating each pair of adjacent lobes. As noted, each aperture 118 corresponds to the cross-sectional shape of the tube that is attached (e.g. by laser welding) at the aperture at the plate through which the aperture extends so that the tube's internal volume is in fluid communication with the aperture. The tube cross-sections are best seen in FIGS. 3 and 5.

Referring to FIG. 1, a first end 144 of each heat exchanger tube 140 is secured to a corresponding entry aperture 118 of first end plate 116, such as by laser welding, in a fluid-tight manner. Similarly, a second end 146 of each heat exchanger tube 140 is secured in alignment with a corresponding exit aperture 124 (which has the same shape as aperture 118) of second end plate 122, such as by laser welding, in a fluid-tight manner. Additionally, first and second end plates 116 and 122 each include an outer perimeter 120 and 126, respectively, that are secured to an inner surface of shell 102 in a fluid-tight manner. As such, first end plate 116, second plate 122, and the portion of shell 102 disposed therebetween define a first volume 112 that is configured to receive a first fluid, such as, but not limited to, water, therein. Similarly, combustion chamber 128, outlet chamber 138, and heat exchanger tubes 140 define a second volume 130 that is configured to receive a second fluid, such as, but not limited to, combustion gasses, therein.

Referring now to FIGS. 3 through 5, each twisted heat exchanger tube 140 includes first end 144, second end 146, and an elongation dimension, e.g. a longitudinal or symmetrical center axis 142, extending therebetween. As previously noted, the cross-section of twisted heat exchanger tube 140, when taken in a plane that is transverse to axis 142, includes four lobes 152, 154, 156 and 158 that extend radially-outwardly from a theoretical cylinder centered about longitudinal center axis 142 and ending at the four corners 149 between the lobes. As shown, lobes 152, 154, 156 and 158 are symmetrically spaced about longitudinal center axis 142 at 90° intervals. In alternate embodiments of twisted heat exchanger tubes, the lobes need not be symmetrically spaced. As well, alternate embodiments of twisted heat exchanger tubes need not have four lobes. For example, as shown in FIG. 6, twisted heat exchanger tube 140 a includes three lobes 152 a, 152 b, and 152 c, which are spaced at 120° intervals about longitudinal center axis 142, forming outer flow paths 162 a, 164 a, and 166 a therebetween.

As shown, twisted heat exchanger tube 140 includes an inner surface 148 and outer surface 150 that are defined by a simultaneous rotation and translation of the tube's four-lobed cross-section along longitudinal center axis 142 so that a line defined by the outermost edge of each lobe, or a line defined by the centroid of the inner volume of each lobe, defines a helical path about center axis 142 between tube ends 144 and 146. Although the helical paths illustrated in the Figures define a uniform pitch, it should be understood that the helical pitch may vary between tube ends 144 and 146, depending on the tube's construction. As best seen in FIG. 4, each twisted heat exchanger tube 140 is formed by at least one full twist portion 170, which is defined by a full rotation of the cross-section about longitudinal center axis 142 through 360° over a given length of twisted heat exchanger tube 140. For example, twisted heat exchanger tube 140 shown in FIG. 4 includes three full twist portions 170, with a dotted line 153 being provided on a first lobe 152 to help visualize rotation of first lobe 152 through a 360° helical pattern about longitudinal center axis 142. As such, for a twisted heat exchanger tube 140 that is 48 inches in length and includes three full twist portions 170, it follows that each full twist portion is sixteen inches in length. Note, however, in alternate embodiments a heat exchanger tube may be formed by less than a full twist portion, or be formed by less than exact multiples of full twist portions, i.e., it may include a partial twist portion. Note, however, that each twisted heat exchanger tube 140 is preferably formed along its entire length by full and/or partial twist portions in order to maximize the inner and outer surface areas for enhanced heat transfer.

As best seen in FIG. 5, outer surface 150 of each twisted heat exchanger tube 140 forms a plurality of outer flow paths 162, 164, 166 and 168, each being disposed between adjacent lobes 152, 154, 156 and 158. The increased area of outer surface 150 provides increased heat transfer characteristics as compared to a cylindrical heat exchanger tube whose cross-section that bounds the same area as the disclosed four-lobe cross-section or that bounds the interior corners between the lobes. Similarly, the increased area of inner surface 148 also provides increased heat transfer characteristics when compared to the noted cylindrical heat exchanger tube.

Referring again to FIGS. 1 and 2, operation of heat exchanger 100, for example a fire-tube boiler, causes heat to be transferred to a first fluid that is passing through first volume 112 of shell 102 from a second fluid that is passing through the plurality of twisted heat exchanger tubes 140. The first fluid, e.g. water, flows into shell 102 and volume 112 at inlet 108, passes over the outer surfaces of the plurality of twisted heat exchanger tubes 140 that extend through first volume 112, and ultimately flows out of shell 102 through outlet 110 as steam or water-carrying steam (i.e. wet steam). The flow of water into, through, and out of first volume 112 of shell 102 is represented by flow arrows 115. Note, multiple inlets 108 and outlets 110 may be provided on shell 102 for the ingress and egress of water. Simultaneously, the second fluid, e.g. hot combustion gas generated by combustion at burner 134, is propelled through second volume 130, which is defined by combustion chamber 128, the inner volume of twisted heat exchanger tubes 140, and outlet chamber 138. To achieve the desired flow of the second fluid, which in the instant case is a hot combustion gas, a fuel is first combusted in combustion chamber 128. Fuels such as, but not limited to, natural gas from a natural gas line or other source (not shown) in communication with burner 134 may be used. The resultant hot combustion gasses are moved from combustion chamber 128 through the plurality of twisted heat exchanger tubes 140 by blower 136. As should be understood, the heat exchange rate between the combustion gas and the tube wall, and therefore between the combustion gas and the water within the tank volume, increases or decreases directly with increases and decreases in the speed at which the gas moves through the tubes. Thus, blower 136 is operated at a level to achieve a desired heat transfer rate between the hot combustion gasses in second volume 130 and the water passing over twisted heat exchanger tubes 140 in first volume 112, as compared to the cost of operating the blower in reaction to flow resistance within the system. As shown, the flow direction of first fluid 115 is substantially counter to that of the second fluid that moves downwardly through twisted heat exchanger tubes 140. After passing through twisted heat exchanger tubes 140, the hot combustion gasses exit heat exchanger 100 by way of outlet 139 of outlet chamber 128.

Referring again to FIG. 5, as the hot combustion gasses flow downwardly through the interior volume of twisted heat exchanger tubes 140, heat is transferred outwardly through the wall of the tube to the cooler water flowing in the opposite direction over the outer surface thereof. As heat is transferred from the flow of hot combustion gasses outwardly through the wall of twisted heat exchanger tube 140, those layers of the flow of hot combustion gasses that are adjacent inner surface 148 (“boundary layers”) begin to cool with respect to the remainder of the flow. As such, the outermost boundary layers become more dense than the remainder of the boundary layers of the hot combustion gas flow. The differences in densities and temperatures between the outermost boundary layers and remaining boundary layers cause small localized currents 170 to develop within the boundary layers adjacent inner surface 148 of heat exchanger tube 140. Currents 170 within these boundary layers cause mixing therebetween, which enhances the heat transfer outwardly through the wall of twisted heat exchanger tube 160 as warmer, less dense combustion gasses move closer to inner surface 148 as the cooler, denser gasses move away from inner surface 148.

Due to the multi-lobe cross-section design of twisted heat exchanger tube 140, the flow mixing that results from currents 170 is enhanced as compared to a cylindrical heat exchanger tube having the same cross-sectional area, as the multiple lobes provide an increased surface area over which currents 170 develop. As well, the twisted configuration of heat exchanger tube 140 increases the effective length of the tube (considered in terms of tube surface area) as compared to a typical non-twisted tube, i.e., the inner and outer surface areas of twisted tube 140 for a given length will be greater than the inner and outer surface areas of the non-twisted tube over the same given length. Therefore, currents 170 develop over a shorter span of the heat exchanger tube 140 than of a cylindrical, non-twisted tube. Currents 170 contribute to flow separation of the boundary layers from the wall of heat exchanger tube 140, which enhances turbulent flow across the entire cross-section thereof. As such, heat transfer along the length of heat exchanger tube 140 is enhanced. Moreover, given that it is desired in a fire tube boiler to convey sufficient heat from combustion gas to the water in volume 112 to convert the water to steam, the inner surface area of the tubes over the same length allows each tube to transfer a greater amount of heat, thereby allowing the use of fewer tubes within the tank and, therefore, a smaller tank to process the same amount of water as could be processed by a tank having cylindrical tubes of the same length.

While one or more preferred embodiments of the invention are described above, it should be appreciated by those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope and spirit thereof. Accordingly, it should be understood that the elements of one embodiment may be combined with another embodiment to create a still further embodiment. It is intended that the present invention cover such modifications and variations as come within the scope and spirit of the present disclosure, the appended claims, and their equivalents. 

What is claimed is:
 1. A heat exchanger, comprising: a shell having a first end and a second end; a first end plate and a second end plate disposed adjacent the first end of the shell and the second end of the shell, respectively, the shell and the first and second end plates defining a first volume configured to receive a first fluid therein; and at least one heat exchanger tube having a first end affixed to the first end plate and a second end affixed to the second end plate, the at least one heat exchanger tube extending through the first volume and including a cross-section taken transverse to its longitudinal center axis that is defined by a plurality of lobes, each lobe extending radially outwardly from the longitudinal center axis thereof, the at least one heat exchanger tube including an inner surface and an outer surface defined by a simultaneous rotation and translation of the cross-section along the longitudinal center axis, wherein the cross-section is constant along an entire length of the at least one heat exchanger tube between the first end plate and the second end plate.
 2. The heat exchange of claim 1; wherein a first fluid passes through the first volume of the shell, and wherein a second fluid comprising a hot combustion gas passes through the at least one heat exchanger tube.
 3. The heat exchanger of claim 1, wherein: the first end plate defines a first aperture having a same shape as the cross-section of the at least one heat exchanger tube; the second end plate defines a second aperture having the same shape as the cross-section of the at least one heat exchanger tube, and the shape of the first aperture and the second aperture conforms to the outer surface of the at least one heat exchanger tube.
 4. The heat exchanger of claim 3, wherein the at least one heat exchanger tube further comprises a plurality of heat exchanger tubes, the cross-section of each including four lobes.
 5. The heat exchanger of claim 4, wherein each heat exchanger tube includes at least a full twist portion in which each lobe thereof rotates about the longitudinal center axis thereof through 360°.
 6. The heat exchanger of claim 4, wherein the first end and the second end of each heat exchanger tube is affixed to the first end plate and the second end plate, respectively, by a weld.
 7. The heat exchanger of claim 4, wherein the cross-section of each heat exchanger tube includes four lobes disposed at 90° intervals about the longitudinal center axis.
 8. The heat exchanger of claim 4, further comprising a combustion chamber disposed at the first end of the shell, the combustion chamber being in fluid communication with the plurality of heat exchanger tubes.
 9. The heat exchanger of claim 8, further comprising a blower in fluid communication with the combustion chamber so that the blower moves the second fluid through the plurality of heat exchanger tubes.
 10. The heat exchanger of claim 4, wherein the second volume of the shell includes at least one inlet disposed at its second end and at least one outlet at its first end so that the first fluid flows counter to the second fluid.
 11. The heat exchanger of claim 4, wherein the first fluid is water.
 12. The heat exchanger of claim 4, wherein a length of each heat exchanger tube is less than 48 inches.
 13. A heat exchanger, comprising: a shell having a first end, a second end, and defining a first volume configured to receive a first fluid; and a first end plate disposed at a first end of the shell, the first end plate defining a first aperture of a first shape; a second end plate disposed at a second end of the shell, the second end plate defining a second aperture of a second shape; at least one heat exchanger tube having a first end affixed to the first end plate at the first aperture and a second end affixed to the second end plate at the second aperture, the at least one heat exchanger tube extending through the first volume and including a cross-section taken transverse to its longitudinal center axis that is defined by at least three lobes, each lobe extending radially outwardly from the longitudinal center axis thereof, the at least one heat exchanger tube defining a second volume, wherein the cross-section is constant along an entire length of the at least one heat exchanger tube between the first end plate and the second end plate, and the first shape and the second shape of the first end plate and the second end plate, respectfully, are the same shape as the cross-section of the at least one heat exchanger tube.
 14. The heat exchanger of claim 13, wherein the at least one heat exchanger tube includes an inner surface and an outer surface defined by a simultaneous rotation and translation of the cross-section along the longitudinal center axis.
 15. The heat exchanger of claim 14, wherein the first aperture and the second aperture conform to the outer surface of the at least one heat exchanger tube.
 16. The heat exchanger of claim 15, wherein the first end and the second end of the at least one heat exchanger tube are affixed to the first and the second end plates, respectively, by welding.
 17. The heat exchanger of claim 14, wherein the at least one heat exchanger tube further comprises a plurality of heat exchanger tubes, the cross-section of each including four lobes disposed at 90° intervals about the longitudinal center axis.
 18. The heat exchanger of claim 17, further comprising a combustion chamber disposed at the first end of the shell, the combustion chamber being in fluid communication with the second volume of the plurality of heat exchanger tubes.
 19. The heat exchanger of claim 18, further comprising a blower in fluid communication with the combustion chamber so that the blower moves the hot combustion gasses through the second volume of the plurality of heat exchanger tubes. 